The present invention relates to an offset correcting method, an offset correcting circuit, and an electronic volume suitable for offset correction of the electronic volume.
In the related art, there is the electronic volume in which the amplifier circuits, etc. are employed as constituent elements and whose gain is controlled by the digital signal. In the situation that DC offset is contained in the output of this electronic volume, DC potential of the output is changed and thus the noise is generated when the gain of the amplifier circuit is varied.
Meanwhile, the offset voltage of the OP amplifier as one of the amplifier circuits is varied depending upon the temperature variation, the age-based change, and the like. For this reason, the correction of the offset voltage executed only in the manufacturing stage of the OP amplifier does not make it possible to correct the offset voltage generated depending upon the temperature variation, the age-based change, and the like. In order to settle this problem, the circuit for correcting the offset voltage at the manufacturing stage et seq. of the OP amplifier was proposed.
As such offset voltage correcting circuit in the related art, the circuit for executing the offset voltage correction by measuring the offset voltage of the OP amplifier and then adding the offset voltage to the input signal of the OP amplifier as a correction value was proposed (see JP-B-2888833, for example).
Further, as the offset voltage correcting circuit in the related art, the circuit for executing the offset voltage correction by adding MOS transistors in series with respective load transistors in the input differential portion of the OP amplifier, then connecting capacitors between the gates and the drains of respective MOS transistors, and then adjusting charge voltages of both capacitors to change gate voltages of the MOS transistors was proposed.
Normally, it is difficult to arrange the capacitor with a large capacitance value in the chip of the OP amplifier. Therefore, in the above offset voltage correcting circuit in the related art, since the electric charge charged in the capacitor is discharged in a short time at the end of the correction, the correction must always be executed in a predetermined period. For this reason, in the related art, the offset voltage correcting circuit for correcting the offset voltage of the OP amplifier by using the comparing element (comparator), which compares the output voltage of the OP amplifier with a predetermined reference voltage, without the capacitor was proposed (see JP-A-11-88071, for example).
However, in the offset voltage correcting circuit set forth in JP-B-2888833, since the correction value is added to the input signal of the OP amplifier, there is a fear that the dynamic range, the distortion factor, input/output impedances, etc. of the OP amplifier are deteriorated. Therefore, when the electronic volume is constructed by using the offset voltage correcting circuit set forth in JP-B-2888833, the dynamic range, the distortion factor, input/output impedances, etc. of such electronic volume may be deteriorated.
In the offset voltage correcting circuit set forth in JP-A-11-88071, in order to eliminate (cancel) the offset of the OP amplifier with high precision, the comparator, etc. for sensing the offset value of the OP amplifier requires the high-precision performance. It is not easy to design and manufacture such high-precision comparator and also a great production cost is needed.
When the offset is contained in the OP amplifier as the constituent element of the electronic volume, the noise is generated due to the offset value. In addition, in the electronic volume having an amplifying function, the noise due to such offset value is amplified in response to the amplification factor and then output. As a result, in the related art, in case it is intended to manufacture the electronic volume that can eliminate the offset of the OP amplifier with high precision and generate no noise, such a problems occurs that it is not easy to design and manufacture such electronic volume and also an enormous production cost is needed.